1. Field of the Invention
The present invention relates to an improvement in a gas analyzer for measuring concentration of specific constituents in the sample gas, particularly to an improvement in a gas analyzer using dual type pyroelectric sensors.
2. Description of the Prior Art
FIG. 1 shows an example of a dual type pyroelectric sensor DP (hereinafter called simply a dual pyroelectric sensor) comprising two pairs of electrodes 2a and 2b and 3a and 3b provided on the front and reverse sides of a single piece of pyroelectrical material 1, all of the abovesaid components being contained in a package 4, the package having an opening covered by a filter 5 serving as a light receiving element in line with one pair of electrodes 2a and 2b, and a further opening covered a radiation blocking plate 6 serving as an auxiliary element in line with the other pair of electrodes 3a and 3b. The dual type pyroelectric sensor having the structure as described has excellent noise characteristics due to temperature changes or vibration as compared with the so-called single type pyroelectric sensor containing no more than one pair of electrodes in a package. Therefore, it is currently favored for use in the sensing part of the gas analyzer.
FIG. 2 and FIG. 3 show an example of a conventional gas analyzer using dual pyroelectric sensors DP as described above.
In FIG. 2, the reference numeral 11 designates a light source emitting infrared rays and reference numeral 12 designates a cell through which infrared rays are projected from the light source 11. The cell is closed at both ends thereof by cell windows 13 and 14 composed of an infrared ray passing material, and is provided with an inlet port 15 and outlet port 16 for introducing and discharging the sample gas.
The reference numeral 17 designates a sensing part for sensing constituents (for example, CO, CO.sub.2, HC, etc.) of the above-said sample gas to be analyzed, and which is disposed to receive infrared rays passing through the cell 12. The numeral 18 designates a chopper between the cell and the sensing part and having a shaft 18' extending to a driving motor (not shown). As shown in FIG. 3, in the sensing part 17, are four dual pyroelectric sensors DP.sub.1, DP.sub.2, DP.sub.3, and DP.sub.4, the first three being for sensing CO, CO.sub.2 and HC, respectively, and the fourth one for reference. The filters 5 of the dual pyroelectric sensors DP.sub.1 through DP.sub.4 are band-pass filters which permit passage of infrared rays of a specific absorption band corresponding to the material to be measured therethrough. For example, the filter 5 of the dual pyroelectric sensor DP.sub.1 for sensing CO is a band-pass filter permitting infrared rays of an absorption band specific to CO to pass therethrough. The filters of the dual sensors DP.sub.2 and DP.sub.3 pass infrared rays specific to CO.sub.2 and HC. On the other hand, the filter of the reference dual pyroelectric sensor DP.sub.4 is a bandpass filter which permits infrared rays of wavelengths lying outside the absorption bands specific to CO, CO.sub.2, and HC to pass therethrough.
In the conventional gas analyzer, as shown in FIG. 3, dual pyroelectric sensors DP.sub.1 through DP.sub.4 are in an arrangement for enabling all light receiving elements and auxiliary elements, as a whole, to be completely encircled by the inner periphery 12' of the cell 12, so that the diameter of the cell is naturally quite large. Accordingly, the internal volume of the cell 12 is large and requires a large volume of the sample gas to fill it, whereby the length of time this gas stays in the cell 12 is prolonged, a period of response time before the appearance of a stable output is also increased, and this in turn causes problems in the response characteristic, the density energy acting upon each light receiving element 2 is low, and the S/N ratio is unsatisfactory.
One conceivable way of coping with the above described problem is to reduce the distance between each pair of dual pyroelectric sensors DP.sub.1 through DP.sub.4 and also to reduce the inner diameter d of the cell. However it is impossible to arrange the dual pyroelectric sensors DP.sub.1 through DP.sub.4 any closer to each than a certain distance because the revolving shaft 18' of the chopper 18 passes through a space encircled by the dual pyroelectric sensors DP.sub.1 through DP.sub.4 so as to be connected to a driving motor (not shown).
Another way is to shorten the length l of the cell 12. However, at some times low concentrations of CO and HC cannot be measured because the length l of the cell is too short.